1. Field of the Invention
The present invention generally relates to the field of implantable cardioverter defibrillators. More specifically, the invention relates to an implantable cardioverter defibrillator (ICD) of the type including a system for automatic capture threshold determination.
2. Description of the Prior Art
Heart defibrillation is currently performed by the discharge of a powerful voltage pulse between two electrodes. The electrodes are placed so the discharge takes place over the heart or a large part thereof. The energy in a pulse amounts typically from a few joules up to a few dozen joules.
In order to reduce pacing energy consumption and increase longevity, methods for automatic capture threshold determination, or automatic regulation of pacing pulse settings, may be provided in cardiac stimulators for maintaining the energy of the stimulation pulses at a level just about what is needed to effectuate capture. One such method is known as the AutoCapture™ Pacing System of the type described in PCT Application WO 99/65566. Longevity is increased because automatic capture detection allows pacing at lower energies without compromising patient safety. It is also known to provide automatic capture pacing in an ICD system, see e.g. U.S. Pat. No. 6,327,498.
The autocapture functionality of the AutoCapture™ Pacing System automatically adapts the stimulation output to the minimal energy required to capture the heart. After each single delivered pacing stimulus, capture is verified. The verification is based on detection of the Evoked Response (ER), which is defined as the electrical response of the myocardium to a pacing stimulus. During a time window after an emitted pacing pulse, sensing circuitry of the heart stimulator looks for an evoked response. If an evoked response is not detected during the detection window, the heart stimulator interprets this as loss of capture and a back-up pulse of higher energy is emitted at the end of the detection window. Conversely, if an evoked response is detected capture is verified.
If two consecutive losses occur, then a threshold search will be performed in order to evaluate whether the pacing threshold has changed. If necessary, the pacing pulse settings will be adjusted such that subsequent pacing pulses will have a higher energy content as compared to the energy content of the two consecutive pulses that did not effect an evoked response. The process of changing the energy content in the pacing pulses is referred to as a threshold search. In this process, the energy content in the pacing pulses is adjusted in steps and capture is verified for each pacing pulse. After the threshold has been determined, a working margin is added to the measured threshold in order to determine the new pacing pulse settings following the threshold search. This working margin is typically in the order of 0.3 V.
The above discussed threshold search is caused by an increased threshold and will be executed immediately when two consecutive losses of capture has been detected. If the pacing threshold is stable for a longer period of time and no sudden changes occur, i.e. there is no consecutive losses of capture, then threshold searches will be initiated by a timer at regular intervals. These intervals are typically in the order of eight hours. This will allow for the pulse generator to adapt to threshold changes regardless of whether the threshold increases or decreases.
The large energy required in a defibrillation shock, e.g. an ICD shock, has shortcomings as regards the longevity of an ICD device. Furthermore, the powerful energy discharge in the ICD shock has adverse effects on the organism. For instance, it is known that the pacing or capture threshold is increased immediately following a defibrillation shock. Furthermore, the length of the depolarization phase may also be affected, as well as the length of the refractory period of the myocardium.
Thus, the results of automatic capture threshold detection, below simply referred to as autocapture, immediately following an ICD shock may differ considerably from the results of autocapture provided when the myocardium is not affected by a recent ICD shock, which may have an adverse effect on the accuracy of the autocapture measurements. For instance, autocapture measurements immediately following the delivery of a shock may result in a higher pacing threshold. The resulting higher pacing output will be used until the next threshold search is initiated by the timer. Furthermore, polarization from the pacing electrode may become modified immediately following the delivery of a shock. This can result in changes in the sensing signal used for detecting capture following a shock.